Since radioactive gas emissions from nuclear power plants are the major contributor to the radiation dose of the population in the vicinity of these facilities, accurate measurements of the radioisotopes and activities in these gaseous emissions are very important. Table 1 is a list of the most important radioactive fission product gases found in gaseous effluents from nuclear power plants. Since the radioactive gas emissions contain a number of different radionuclides, an accurate identification and quantitative determination of the isotopes in the effluent must be performed using a gamma spectrometer system. Present day systems generally use high resolution semiconductor detectors rather than the poorer resolution NaI(Tl) detectors of earlier systems. The increased resolution of the modern semiconductor detectors has greatly improved the qualitative aspect of radioactive gas measurements. However, the wide variations in configuration and internal dimensions of the active volume of semiconductor detectors has increased the difficulty in performing quantitative measurements. The variation of detector efficiency with gamma ray energy, source shape, and source position relative to the detector must be measured for each individual detector. As shown in Table 1 the energy range of greatest interest in gaseous fission product monitoring is the region from 81 keV to 514 keV. The very rapid change in detector efficiency in this energy range requires that many efficiency measurements be performed for an accurate system calibration.
Many different types of containers are used in power plant laboratories to trap radioactive gases and hold them while radioactivity measurements are being made. Since the concentration of radioactive gas varies over several orders of magnitude depending on the position of collection in the power plant, a wide range of sizes of gas counting containers must be available. For low activity samples large containers of one to nine liters are commonly used in order to measure the concentration required by current regulations. High activity samples must be measured in small containers at large distances from the radiation detector so as not to exceed the count rate limitations of spectrometer systems. Radioactive gas containers are made from a variety of materials. Plastics are inexpensive but have the disadvantage that radioactive noble gases diffuse through them in a matter of days. Metal containers retain the radioactive gases very well but are expensive and in general scatter and attentuate the gamma radiation to a greater degree than the plastic containers. In spite of their fragile nature, glass containers are frequently used in radioactive gas measurements. Glass containers vary in their ability to retain noble gases depending on the method used to seal them. Stopcocks made of glass can retain noble gases if they are properly used; and stopcocks are, in general, superior to rubber septum closures for gas retention. Each nuclear power plant with its unique effluent train environment and economic conditions uses a different combination of radioactive gas containers in its measurement process. The gamma spectrometer system used to assay radioactive gases must be calibrated to obtain the counting efficiency as a function of gamma ray energy for each separate container and each counting position before quantitative measurements can be performed.
TABLE 1 ______________________________________ RADIOACTIVE FISSION PRODUCT GASES Isotope Half-Life Principal Gamma Ray Energies (keV) ______________________________________ Kr-85 m 4.48 h 149.5, 305 Kr-85 10.76 y 514 Kr-87 76.4 m 403 Kr-88 2.80 h 196, 2392 Xe-133 m 2.18 d 232.8 Xe-133 5.27 d 81 Xe-135 9.16 h 249.6 Xe-137 3.82 m 455.6 Xe-138 14.2 m 258.2, 434.4, 1769, 2013 ______________________________________